The present invention relates to an electron-emitting source comprising a plurality of cylindrical carbon nanotubes which are formed by chemical vapor deposition on a substrate containing iron or the like, and a method of manufacturing the same.
A carbon nanotube comprises a completely graphitized cylinder having a diameter of about 4 nm to 50 nm and a length of about 1 μm to 10 μm. Examples of the carbon nanotube include one having a shape in which a single graphite layer (graphene) is closed cylindrically and one having a shape in which a plurality of graphenes are layered telescopically such that the respective graphenes are closed cylindrically to form a coaxial multilayered structure. The central portions of the cylindrical graphenes are hollow. The distal end portions of the graphenes may be closed, or broken and accordingly open.
It is expected that the carbon nanotube having such a specific shape may be applied to novel electronic materials and nanotechnology by utilizing its specific electronic physical properties. For example, the carbon nanotube can be used to form an emitter which emits electrons. When a strong electric field is applied to the surface of a solid, the potential barrier of the surface of the solid which confines electrons in the solid becomes low and thin. Consequently, the confined electrons are emitted outside the solid due to the tunnel effect. This phenomenon is so-called field emission.
In order to observe field emission, an electric field as strong as 107 V/cm must be applied to the solid surface. As a scheme to implement this, a metal needle with a sharp point is used. When an electric field is applied by using such a needle, the electric field concentrates at the point of the sharp needle, and a necessary strong electric field is obtained. The carbon nanotube described above has a very sharp point with a radius of curvature on the nm order, and is chemically stable and mechanically tough, thus providing physical properties that are suitable as a field emission emitter material.
When the carbon nanotube having the characteristic feature as described above is to be used in an electron-emitting source in an FED (Field Emission Display) or the like, carbon nanotubes must be formed on a substrate having a large area. Carbon nanotube manufacturing methods include electric discharge in which two carbon electrodes are set apart from each other by about 1 mm to 2 mm in helium gas and DC arc discharge is caused, laser vapor deposition, and the like. With these manufacturing methods, however, the diameter and length of the carbon nanotube are difficult to adjust, and the yield of the carbon nanotube as the target cannot be increased very much. A large amount of amorphous carbon products other than carbon nanotubes are produced simultaneously. Thus, a purifying process is required, making the manufacture cumbersome.
In order to solve the above problems, as shown in Japanese Patent Laid-Open No. 2001-048512, a method has been proposed in which a catalyst metal layer is prepared on a substrate and, while the substrate is heated, carbon source gas is supplied onto the catalyst metal layer to grow a large number of carbon nanotubes from the catalyst metal layer. In the manufacture of the carbon nanotube according to this thermal chemical vapor deposition (CVD) method, the length and diameter of the carbon nanotube to be formed can be controlled depending on the type of the catalyst metal, the duration of growth, the type of the substrate, and the like.
When a carbon nanotube is to be used as an electron-emitting source, if a thinner carbon nanotube is used, electrons can be emitted with a lower voltage. For example, when a carbon nanotube is to be used as an electron-emitting source in an FED, if a thinner carbon nanotube is used, driving is enabled at a lower voltage. This is preferable in terms of power saving.
When a carbon nanotube is to be formed by CVD, a plurality of carbon nanotubes can be formed close to each other on a substrate. When the temperature of the substrate is set to a high temperature of 800° C. to 1,000° C., thin carbon nanotubes having diameters of about 10 nm can be formed. If, however, an electrode structure which forms a cathode is formed of a catalyst metal and carbon nanotubes are to be directly formed on the electrode structure made of the catalyst metal so that the carbon nanotubes can be used as an electron-emitting source, the following problems arise.
In the above arrangement, as the electrode structure surface (growth surface) where the carbon nanotubes grow is entirely made of the catalyst metal, the carbon nanotubes can grow at any portion of the growth surface. Therefore, when the carbon nanotubes are to be formed by CVD, they can readily grow from the growth surface thickly with no gaps among them. When a plurality of thin carbon nanotubes are to be formed by CVD, the plurality of carbon nanotubes that are adjacent to each other tend to come into contact with each other to form a bundle. Then, it is very difficult to form carbon nanotubes evenly on the entire region of the substrate. If the carbon nanotubes are not formed evenly, field emission tends to occur locally. Local field emission may break a carbon nanotube and, depending on the case, cause a chain reaction to break many carbon nanotubes.
It is known that, when the temperature of the substrate is set to a comparatively low temperature of about 600° C. to 700° C., even if the growth surface is entirely made of the catalyst metal, a plurality of carbon nanotubes are formed more evenly such that they will not easily form bundles. The carbon nanotubes formed in this manner, however, are comparatively thick with diameters of 40 nm, and require a higher driving voltage than thin carbon nanotubes. This is not preferable in terms of power saving.